fork: Remove the weak insanity

We error out when compiling with gcc4.1.[01] as it miscompiles
__weak. The workaround with magic defines is not longer
necessary. Make it __weak again.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20120505150141.306358267@linutronix.de